Display tube provided with an interference filter

ABSTRACT

A display tube comprising in an evacuated envelope (1) a display screen (7) provided on the inside of a display window (2) in the wall of the envelope (1), which display screen (7) comprises luminescent material (13), and a multilayer interference filter (12) is provided between this material and the display window and comprises a number of layers (HL) which alternately are manufactured from a material having a high (H) and a material having a low (L) refractive index. If the filter is composed substantially of 6 to 30 layers, each having an optical thickness nd, wherein n is the refractive index of the material and d is the thickness, which optical thickness is between 0.2λ f  and 0.3λ f  and preferably between 0.23λ f  and 0.25λ f , wherein λ f  is equal to p xλ, wherein λ is the desired central wavelength which is selected from the spectrum emitted by the luminescent material (13) and p is a number between 1.15 and 1.45, a display tube is obtained in which the luminous efficiency in the forward direction is at least 40% larger than in a tube without such a filter. Color improvement and gain in contrast also occur.

The invention relates to a display tube comprising in an evacuated envelope a display screen which is provided on the innerside of a display window in the wall of the envelope, which display screen comprises luminescent material, and a multilayer interference filter is provided between said material and the display window, said filter comprising a number of layers manufactured alternately from a material having a high refractive index and a material having a low refractive index.

Display tubes have a wide field of application. They are used, for example, as projection television display tubes, oscilloscope tubes, X-ray amplifier tubes, D.G.D.-tubes (DGD=Data Graphic Display), or as television display tubes.

Such a display tube is known from German Patent Application No. 2330898 laid open to public inspection, in which a multilayer interference filter is used between the luminescent material (the phosphor) and the display window. For the layer thicknesses for the layers of the filter from a material having a high refractive index an optical thickness smaller than 0.25λ or between 0.5 and 0.75λ is chosen, wherein λ is the wavelength of the light emitted by the luminescent material. For the layers of the filter from a material having a low refractive index an optical thickness of 0.25λ is used or an odd multiple thereof. The light reflection characteristic of the display screen of the tube is varied by the filter so that the quantity of light reflected within a limited angular aperture increase by 25% as compared with a tube without a filter. At the same time the contrast in the produced picture is increased by a reduction of the background brightness.

A similar tube is disclosed in British Patent Specification No. 1,306,335 in which a so-called pass-band interference filter is provided between the phosphor and the display screen. This filter consists of layers having an optical thickness of 0.25λ and has the following composition:

S L H L L L L L L H L H L L L L L L H L H L L L L L L H,

wherein S is the display screen, L are layers of a material having a low refractive index and H are layers from a material having a high refractive index.

Such a filter may also be written as follows:

S L H (L)⁶ H L H (L)⁶ H L H (L)⁶ H.

As a result of the presence of the six layers placed against each other and having a low refractive index (L)⁶, a Fabry-Perot filter is formed. With a given choice of the optical layer thickness, said filter has a desired transmission band for light rays which make an angle smaller than 25° to 35° to the normal on the filter. In addition, however, there is an undesired wide transmission band for light rays which enclose an angle between 55° and 90° to the normal. All the light which is passed in this band is lost or contributes to the so-called halo and as a result of this to loss of contrast in the displayed picture. This will be explained in detail hereinafter. The cryolite layers used in this filter moreover are hygroscopic and remain soft, so that damage to the filter can easily occur. The zinc sulphide layers (ZnS) used in this filter easily oxidize when the tube is fired at approximately 475° C., so that they are less suitable for use in display tubes.

It is therefore an object of the invention to provide a display tube having a multilayer interference filter in which the total quantity of light produced by the tube within an angular aperture of 25° to 35° to the normal is at least 40 to 50% larger as compared with a similar tube without a filter.

Another object of the invention is to indicate such a special choice of the multilayer interference filter that no second transmission band occurs for light rays which make an angle between 35° and 90° to the normal on the filter.

A further object of the invention is to provide a display tube having a multilayer interference filter in which it is possible to realize a colour improvement and a greater contrast in the displayed picture as compared with the known tubes.

An additional object of the invention is to provide a display tube having a multilayer interference filter which can be fired at 440° to 460° C.

According to the invention, a display tube of the kind described in the opening paragraph is characterized in that the filter is composed substantially of 6 to 30 layers each having an optical thickness nd, wherein n is the refractive index of the material of the layer and d is the thickness, said optical thickness nd being between 0.2λ_(f) and 0.3λ_(f) wherein λ_(f) is equal to pxλ, in which λ is the desired central wavelength selected from the spectrum emitted by the luminescent material and p is a number between 1.15 and 1.45. Said optical thickness nd being preferably between 0.23λ_(f) and 0.25λ_(f). The outermost layers of the filter are preferably manufactured from a material having a high refractive index.

Hence the filter consists only or substantially only of layers having an optical thickness of approximately 0.25λ_(f). As a result the filter possesses the special property of having a very wide reflection band (no transmission) for light rays which make an angle between approximately 30° and 90° to the normal on the filter.

For comparison it should be noted that example, in 0.75λ_(f) and 1.25λ_(f) filters the width of the reflection band is reduced by a factor of 3 and 5, respectively, and undesired transmission occurs with large angles. As a result of this, the gain in luminous efficiency in the forward direction in such filters, hence for light rays which make small angles to the normal on the filter, as, for example, in German Patent Application No. 2330898, is restricted to 25%. There is also less gain in contrast. The Fabry-Perot filter according to British Patent Specification No. 1,306,335 comprises three 1.5λ layers (each consisting of six 0.25λ layers of a material having a low refractive index) and also has an undesired wide transmission band for light rays which make large angles to the normal on the filter.

The layers in the filter according to the invention are hence chosen to be so thick that at a desired wavelength the reflection begins to increase considerably for light rays from a luminescent material which make an angle exceeding 20° to 50°, preferably larger than 25° to 35°, to the normal on the filter. The wide reflection band ensures that light rays having angles larger than 25° to 35° are reflected as much as possible and after scattering in the luminescent material again get a chance to emanate from the tube in the forward direction, i.e. within an angle of at most 25° to 35° to the normal on the filter. As a result of this a maximum luminous efficiency in the forward direction is obtained which is at least 40% larger than without using the filter. Moreover, the wide reflection band ensures a considerably reduced halo-effect and less light emanating at large angles. The optical thickness nd of the layers of the filter preferably is not the same for all layers but varies between 0.2λ_(f) and 0.3λ_(f), and preferably between 0.23λ_(f) and 0.25λ_(f), as will be explained in greater detail with reference to an embodiment. As a result of this variation of the thickness, a flatter response curve of the filter is obtained. Because the response curve of the filter depends on the wavelength, it is possible, by a combination of a phosphor and an adapted filter, to improve the color point of the light rays which pass through the filter within an angle of 25° to 35° to the normal on the filter.

The value of the number p is between 1.15 and 1.45, dependent on the refractive indices of the layers used and on the width of the selected wavelength range, which will be explained with reference to the specific examples.

A first preferred embodiment of a display tube according to the invention is characterized in that the luminescent material comprises a terbium-activated, substantially green-luminescing phosphor and λ=545 nm and p is a number between 1.2 and 1.4. Terbium-activated, substantially green-luminescing phosphors are, for example, yttrium aluminium garnet-terbium (YAG:Tb), Yttrium silicate-Terbium (Y₂ SiO₅ :T_(b)), Lanthanum oxide bromide-Terbium (LaOBr:Tb), Lanthanum oxychloride-Terbium (LaOCl:Tb), and Indium borate-Terbium (InBO₃ :Tb). The central wavelength λ for al these Tb-activated green phosphors=545 nm. By choosing for the filter according to the invention p between 1.2 and 1.4, hence λ_(f) between 650 and 765 nm, a filter is obtained having a high transmission (exceeding 90%) for light rays which make an angle smaller than 25° to 35° to the normal on the filter. For light rays which make an angle larger than 25° to 35° to the normal on the filter, the transmission declines rapidly and up to 90° reflection occurs. This filter operates particularly well with these Tb-activated phosphors which have emission lines in the blue, green, orange-red and red. By using the filter a gain occurs in the green, a much smaller gain in the blue and a substantially complete reduction of the quantity of light radiated by the tube in the red, which is favourable for the color point. All this provides a luminous efficiency, colour improvement (closer to the EBU standards) and gain in contrast. In an optimum filter with these Tb-activated phosphors the total gain in luminous efficiency for all colours together is 40 to 60%. The gain in the green is 60 to 100%. The coordinates x and y in the CIE color triangle then are 0.27 to 0.29 and 0.60 to 0.64, respectively. Without a filter these coordinates are x=0.33 to 0.36 and y=0.54 to 0.59, respectively. The gain in contrast is approximately a factor 2. For optical display with lenses, as in the case of projection television, this presents important advantages, much less chromatic aberration (little or no red and proportionately less blue as compared to the green).

A second preferred embodiment of a display tube in accordance with the invention is characterized in that the luminescent material comprises a terbium-activated blue-luminescing phosphor with λ=490 nm and p is a number between 1.15 and 1.35. By using a small quantity of Tb for the doping, a predominantly blue-luminescing phosphor with λ=490 nm is obtained. By choosing p between 1.15 and 1.35, hence λ_(f) between 560 nm and 660 nm, a filter is obtained in which the reflection for light rays which make an angle exceeding 25° to 35° to the normal on the filter, increases comparatively steeply. Then there is a gain in contrast of 70% to 100%, and a gain in luminous efficiency in the blue of 50 to 90%. In this case there is a considerable color point improvement because the green spectral lines are filtered away by the multilayer filter.

A third preferred embodiment of a display tube in accordance with the invention is characterized in that the luminescent material is europium-activated Yttrium oxide (Y₂ O₃ :Eu) and λ=612 nm and p is a number between 1.2 and 1.4. As a result of this choice of p between 1.2 and 1.4, hence λ_(f) between 730 nm and 860 nm, the reflection of the filter for light rays which make an angle exceeding 25° to 35° to the normal on the filter increases considerably. Now there is a gain of 60 to 100% in overall luminous efficiency. The gain in contrast is approximately 100%. In this case there is a small color point improvement. The red components in the spectrum of the generated light are actually intensified more than the components of shorter wavelength.

The invention may also be used in wide-band phosphors for example, Willemite (Zn₂ SiO₄ :Mn), zinc sulphide-silver (ZnS:Ag) and strontium aluminate-europium (SrAl₃ O₅.5 :Eu), with central wavelengths λ of 530 nm, 460 nm, respectively. The value of p is now chosen between 1.25 and 1.45.

In all the embodiments the layers of the filter with a high refractive index preferably consist of titanium oxide (TiO₂) or tantalum oxide (Ta₂ O₅). The layers having a low refractive index preferably consist of silicon dioxide (SiO₂) or magnesium fluoride (MgF₂). These layers are preferably provided by vapour deposition. The vapour deposition of the MgF₂ must be carried out warm (approximately 300° C.) to obtain a sufficiently hard layer.

A fourth preferred embodiment of a display tube in accordance with the invention is characterized in that the outermost approximately 0.25λ_(f) -thick layer of the filter facing the lumincescent material consists of a material having a high refractive index and is coated with a terminating layer with an optical thickness of approximately 0.125λ_(f) of a material having a lower refractive index, on which terminating layer the luminescent material is provided.

A fifth preferred embodiment of a display tube in accordance with the invention is characterized in that the outermost approximately 0.25λ_(f) -thick layer of the filter facing the display window consists of a material having a high refractive index and is coated with an intermediate layer having an optical thickness of approximately 0.125λ_(f) of a material having a lower refractive index. By providing a terminating layer and/or an intermediate layer having an optical thickness of approximately 0.125λ_(f) at the phosphor side and/or the display window side, respectively, the transmission in the forward direction for light rays which enclose small angles to the normal is increased and the oscillations in the transmissions are reduced. The response curve becomes flatter.

A sixth preferred embodiment of a display tube in accordance with the invention is characterized in that the outermost approximately 0.25λ_(f) -thick layer of the filter facing the luminescent material is composed of a mixture of metal oxides, which mixture comprises titanium oxide (TiO₂) and a metal oxide from the group beryllium oxide (BeO), magnesium oxide (MgO) and calcium oxide (CaO), in which mixture the quantity of titanium oxide is 70 to 99% by weight.

By using such an extra smooth, hence less porous coating layer on the phosphor side of the filter, the optical contact with the phosphor is reduced so that the gain in contrast and luminous efficiency is extra increased.

Tubes according to the invention are coupled to a system of lenses, preferably optically via liquid, or are provided with liquid cooling in which the coating glass on the outside is provided with an antireflection layer. As a display tube may be used a substantially flat window as is commonly used in projection television display tubes. However, a display window which is curved inwardly, hence towards the vacuum side, may also be used advantageously. This advantage is twofold. First, the requirements imposed on the lenses placed before the display window are reduced. Second, the angular variation with respect to the normal on the display window of the emitted light which is captured by the lense is reduced, so that the factor p in λ_(f) =pλ may be chosen to be slightly smaller and so that extra luminous efficiency at small angles is obtained.

As a display tube may optionally also be used a window of glass fibre optics as described in U.S. Pat. No. 3,226,589 which may be considered to be incorporated herein.

Display tubes according to the invention may be used effectively in projection television devices. The very bright image emitted in the forward direction is captured substantially without losses by a system of lenses having a restricted acceptation angle of, for example, 25° to 30°. As a result of this, a brighter picture with more contrast and an improved colour rendition is obtained.

The invention will now be described in greater detail, by way of example, with reference to a drawing, in which:

FIG. 1 is an elevation, partly broken away, of a projection television display tube,

FIG. 2 is a sectional view of a part of the display screen, filter and display window,

FIG. 3 shows diagrammatically the composition of a filter as used in a display tube according to the invention,

FIG. 4 shows a spectrum of LaOCl:Tb,

FIG. 5 shows the transmission T as a function of the angle α with respect to the normal for light rays through a filter as shown in FIG. 3 for a display tube according to the invention,

FIG. 6 shows the transmission T for perpendicularly incident light rays (0°) as a function of the spectral wavelength λ_(s) for the filter shown in FIG. 3,

FIG. 7 shows the angular distribution of the emitted light (dφ/dx) as a function of the angle α measured on a projection television display tube, and

FIG. 8 shows the transmission T as a function of the angle α for a 12-layer-filter and a 20-layer-filter.

FIG. 1 is a perspective view, partly broken away, of a projection television display tube according to the invention. It comprises in a glass envelope 1 which consists of a display window 2, a cone 3 and a neck 4, in said neck an electron gun 5 for generating an electron beam 6. Said electron beam 6 is focused on a display screen 7 to form a spot 8. The display screen 7 is provided on the inside of the display window 2. The electron beam is deflected over the display screen 7 in two mutually perpendicular directions x, y by means of the system of deflection coils 9. The tube comprises a cap 10 having connection pins 11.

FIG. 2 is a diagrammatic sectional view of a part of a display window 2, the multilayer interference filter 12 and the display screen 7. The display screen again consists of a layer of luminescent material (phosphor) 13 and a thin aluminium film 14 (the so-called "aluminium backing").

FIG. 3 shows diagrammatically a 20-layer-filter 12 between a display screen 7, consisting of a layer of phosphor 13 (Ph) and an aluminium film 14, and the display window 2 (S). The filter consists of layers SiO₂ (refractive index n=1.40) indicated by a letter L and L', and layers of TiO₂ (n=2.30) indicated by a letter H. The layers have a thickness of approximately 0.25λ_(f). The last approximately 0.25λ_(f) thick layer 15 on the side of the display screen 7 is coated with an approximately 0.125λ_(f) thick terminating layer 16 (L').

The phosphor 13 is a Tb-phosphor with λ=545 nm. At p=1.31, λ_(f) becomes equal to 710 nm. The filter composition at this λ_(f) is recorded in the following table.

    ______________________________________                                         Layer No.         n      n · d/λ.sub.f                         ______________________________________                                         Phosphor                                                                        1                L      0.125                                                  2                H      0.249                                                  3                L      0.246                                                  4                H      0.243                                                  5                L      0.240                                                  6                H      0.238                                                  7                L      0.236                                                  8                H      0.235                                                  9                L      0.234                                                 10                H      0.234                                                 11                L      0.233                                                 12                H      0.234                                                 13                L      0.234                                                 14                H      0.235                                                 15                L      0.236                                                 16                H      0.238                                                 17                L      0.240                                                 18                H      0.243                                                 19                L      0.246                                                 20                H      0.249                                                 Display window    1.57   --                                                    ______________________________________                                    

FIG. 4 shows the spectrum (the intensity standardized on the highest peak (I_(p)) in % as a function of λ_(s)) of lanthanum oxychloride-Terbium. This spectrum is representative of all green terbium phosphors, for example,

yttrium aluminium garnet-terbium (YAG:Tb)

yttrium silicate-terbium (Y₂ SiO₅ :Tb)

Lanthanum oxybromide-terbium (LaOBr:Tb) and

indium borate-terbium (InBO₃ :Tb).

FIG. 5 shows the transmission T (solid line) of the filter shown in FIG. 3 as a function of the angle α with respect to the normal on the filter. The angle α_(50%), at which the transmission of the filter has decreased to 50%, is approximately 32°. For comparison, this Figure shows the transmission (broken line) as a function of the angle α for the Fabry-Perot filter according to the said British Patent Specification No. 1,306,335. In order to make this filter comparable with the filter shown according to the invention, α_(50%) for this filter has also been chosen to be 32°. Moreover, the materials have been chosen to be the same, namely TiO₂ and SiO₂. Due to the different character of the filter, p=1.02 in this case, hence λ_(f) =1.02×545=555 nm. A great disadvantage of this filter is the wide transmission band between 55° and 90°. All the light which is transmitted in this transmission band is lost or, after reflection, contributes to halo and thus gives loss of contrast. This disadvantage also applies to 0.75λ_(f) and 1.25λ_(f) filters.

FIG. 6 shows the transmission T for light rays which are incident perpendicularly on the filter (0°) as a function of the spectral wavelength λ_(s) of the FIG. 3 filter. The low pass character of the filter (short-wave-pass-filter) is quite obvious and so is the wide reflection band between 570 and 815 nm.

FIG. 7 shows the light flux φ as a function of the angle (dφ/dx) in arbitrary units as measured in a projection television display tube having a 20-layer-filter as shown in FIGS. 3, 5 and 6. For α smaller than 32° there is a clear gain in luminous efficiency (curve I) with respect to a projection television display tube without a filter (curve II).

FIG. 8 shows the transmission T as a function of the angle α for a 12-layer-filter (curve I) and a 20-layer-filter (curve II for yttrium oxide-Europium with λ=612 nm. The 20-layer-filter is identical to the FIG. 3 filter, except for λ. For p=1.31λ_(f) =1.31×612=802 nm. _(50%) =32°. The 12-layer-filter looks as follows with p=1.31, λ_(f) =802 and α_(50%) =31°.

    ______________________________________                                         Layer No.         n      n · d/λ.sub.f                         ______________________________________                                         Phosphor                                                                       1                 L      0.125                                                 2                 H      0.249                                                 3                 L      0.244                                                 4                 H      0.240                                                 5                 L      0.237                                                 6                 H      0.235                                                 7                 L      0.233                                                 8                 L      0.235                                                 9                 L      0.237                                                 10                H      0.240                                                 11                L      0.244                                                 12                H      0.249                                                 Display window    1.57   --                                                    ______________________________________                                    

The table below finally gives a number of other embodiments and their results. Column 1 gives the kind of phosphor used for the display screen. Column 2 gives the central wavelength of said phosphor. Column 3 gives a few values of p. Column 4 gives the value of α for which the transmission T=50%. Column 5 gives the number of layers of the filter used.

    ______________________________________                                                                      α 50%                                                                            number of                                 Kind of phosphor                                                                           λ (nm)                                                                           p       (in degrees)                                                                           layers filter                             ______________________________________                                         blue-Tb     490      1.27    18      20                                                             1.31    32                                                                     1.34    43                                                green-Tb    544      1.27    18      20                                                             1.31    32                                                                     1.34    43                                                Y.sub.2 O.sub.3 :Eu                                                                        612      1.27    18      20                                                             1.31    32                                                                     1.34    43                                                Willemite   528      1.40    50      12                                        Y.sub.2 O.sub.3 :Eu                                                                        612      1.31    31      12                                        ______________________________________                                     

What is claimed is:
 1. A display tube comprising in an evacuated envelope a display screen which is provided on the inside of a display window in the wall of the envelope, which display screen comprises luminescent material, and a multilayer interference filter is provided between said material and the display window, said filter comprising a number of layers manufactured alternately from a material having a high refractive index and a material having a low refractive index, characterized in that the filter is composed substantially of 6 to 30 layers each having an optical thickness nd, wherein n is the refractive index of the material of the layer and d is the thickness, said optical thickness nd being between 0.2λ_(f) and 0.3λ_(f), in which λ_(f) is equal to pxλ, in which λ is the desired central wavelength selected from the spectrum emitted by the luminescent material and p is a number between 1.15 and 1.45.
 2. A display tube as claimed in claim 1, characterized in that the optical thickness nd is between 0.23λ_(f) and 0.25λ_(f).
 3. A display tube as claimed in claim 1, characterized in that the luminescent material comprises a terbium-activated, substantially green luminescing phosphor and λ=545 nm and p is a number between 1.2 and 1.4.
 4. A display tube as claimed in claim 1, characterized in that the luminescent material comprises a terbium-activated blue luminescing phosphor and λ=490 nm and p is a number between 1.15 and 1.35.
 5. A display tube as claimed in claim 1, characterized in that the luminescing material is europium-activated yttrium oxide (Y₂ O₃ :Eu) and λ=612 nm and p is a number between 1.2 and 1.4.
 6. A display tube as claimed in claim 1, characterized in that the layers of the filter having a high refractive index consist of titanium oxide (TiO₂) or tantalum oxide (Ta₂ O₅) and the layers having a low refractive index consist of silicon oxide (SiO₂) or magnesium fluoride (MgF₂).
 7. A display tube as claimed in claim 6, characterized in that the magnesium fluoride is provided at approximately 300° C.
 8. A display tube as claimed in claim 1, characterized in that the layer of the filter facing the luminescent material is approximately 0.25λ_(f) thick, consists of a material having a high refractive index and is coated with a terminating layer having an optical thickness of approximately 0.125λ_(f) of a material having a lower refractive index, the luminescent material being provided on said terminating layer.
 9. A display tube as claimed in claim 1, characterized in that the layer of the filter facing the display screen has a thickness of approximately 0.25λ_(f), consists of a material having a refractive index and is coated with an intermediate layer having an optical thickness of approximately 0.125λ_(f) of material having a lower refractive index.
 10. A display tube as claimed in claim 1, characterized in that the layer of the filter facing the luminescent material has a thickness of approximately 0.25λ_(f) and is composed of a mixture of titanium oxide (TIO₂) and a metal oxide selected from the group consisting of beryllium oxide (BeO), magnesium oxide (MgO), and calcium oxide (CaO) and in which mixture titanium oxide is present in 70 to 99% by weight.
 11. A display tube as claimed in claim 1, characterized in that the display window is surved inwardly.
 12. A display tube as claimed in claim 1, characterized in that it is a projection television display tube.
 13. A projection television device comprising at least a display tube as claimed in claim
 1. 14. A display window having a multilayer interference filter and a display screen for use in a display tube as claimed in claim
 1. 15. A projection television device comprising a display tube as claimed in claim
 3. 16. A projection television device comprising a display tube as claimd in claim
 4. 17. A projection television device comprising a display tube as claimd in claim
 5. 